Habituation-defined as the decrease in response to the same information over time-is a fundamental process by which we learn about the environment around us. Humans must continually process vast amounts of incoming sensory information. Habituation is thought to play a role in filtering incoming information. As an individual learns that a stimulus in the environment is neither threatening nor rewarding, the stimulus becomes safe and familiar, resulting in habituation at both the behavioral and neural levels. However, failure to habituate to non-threatening stimuli in the environment may trigger feelings of uncertainty or unfamiliarity, resulting in fear and anxiety. Evidence in shy individual supports the hypothesis that habituation may be related to increased social fearfulness. For example, shy individuals are typically slow to acclimate to new people and objects, consistent with slower habituation. Additionally, our lab has recently shown that shy individuals fail to show habituation to novel faces in the amygdala and hippocampus, two brain regions associated with fear and evaluation of threat. However, no studies to date have been conducted exploring habituation in individuals ranging from low to high in social fearfulness. Because individual differences in habituation to social stimuli may provide an important neurobiological marker for risk for psychiatric illness, such as social phobia, we propose that exploration of habituation in individuals ranging from low to high in social fearfulness is critical. In order to study how the rte of habituation contributes to social fearfulness, we will recruit a group of individuals that range from low to high social fearfulness. Using functional MRI, we will measure neural habituation to repeated presentations of neutral human faces. We hypothesize that social fearfulness will be inversely correlated with habituation; individuals with higher levels of social fearfulness will fal to show habituation in several key brain regions that are involved in the expression of fear, including the amygdala, the hippocampus, the ventral and medial prefrontal cortex, and the fusiform face area. However, brain regions do not work in isolation; rather, processing of potential threats occurs over many brain regions that comprise a complex functional network. Therefore, we will also explore functional connectivity across each of these brain regions. As each of these key threat processing brain regions are both structurally and functionally connected, we hypothesize that habituation patterns will be reflected across regions within this network. Specifically, we hypothesize that prolonged communication (failure to habituate) between regions when viewing faces will be associated with higher levels of social fearfulness.